The present invention relates to optical polarizers, in particular, to an inorganic visible light polarizer.
In the liquid crystal display (LCD) industry, particularly for projection color television applications, it is required to polarize the light of the projection light source prior to its impingement on the LCD panel that in turn blocks or transmits the light via selective polarization by sub-pixel cells in the LCD panel. The transmitted light passes through the sub-pixel color filters thus creating a projected color image. Recently, there has developed a trend towards larger screen sizes in the projection color television technology. This, in turn, has created a need for more intense light sources to maintain sufficiently bright projected images on these larger screens. The use of more intense light sources on the other hand, places such a high energy load (thermal and photonic), on the light polarizer that organic polarizers, which are sometimes used in projection televisions, degrade over time leading to degradation of image quality. In order to address this problem, pre-polarizers made of inorganic materials have been placed between the light source and the main (organic) polarizer. The inorganic pre-polarizer is able to withstand the increased energy of the light source while at the same time reducing the load, which is placed on the organic polarizer.
Various types of polarizers are known, including, dichroic, wire grid, anisotropic crystal, as well as reflective and transmissive Brewster""s angle polarizers. The differences between these polarizers are described in High-quality Brewster""s angle polarizer for broadband infrared application, APPLIED OPTICS, Vol. 37, No. 7, pp. 1194-1204 (Mar. 1, 1998). As described therein, Brewster angle polarizers rely on the different transmission and reflection coefficients for s- and p-wave polarization at the interface between two materials. The Brewster angle of a polarizer is the angle of maximum polarization with respect to the incident radiation, and depends on the angle of incidence and the refractive index of the material.
So-called Brewster-angle polarizers use transmissive substrates with multiple coatings made of optically transparent dielectric materials that are alternated with material of different indicies of refraction (e.g., alternate layers of magnesium fluoride and zinc sulfide). In many of these polarizers, light is incident on the dielectric coating and the Brewster angle is dependent on the refractive index of the dielectric material. One illustrative example of a multi-layered dieletric polarizer is described in U.S. Pat. No. 4,515,441, which discloses applying to each of two surfaces of a fused quartz substrate, three-layers of dieletric material. The resulting dieletric polarizer operates at the Brewster angle of the dielectric layer. In addition to other problems, a drawback of multi-layered dielectric polariziers is that they suffer damage due to localized heating in the multiple layers caused by multiphoton absorption. For example, in U.S. Pat. No. 4,553,822, a Brewster angle polarizer is described which uses multiple layers of dielectric material sandwiched between two transparent substrates. Other types of inorganic polarizers for visible light, such as Glan-Thompson calcite prisms or broadband splitter cubes, have not found widespread application in the LCD industry.
Therefore, there continues to be a need for the development of durable and effective inorganic, visible light polarizers. Accordingly, it is the object of the present invention to provide a simple and effective, visible light polarizer.
The present invention relates to the design and production of a polarizer unit, which exhibits effective polarization over the entire visible spectrum. The polarizer unit includes a transparent, inorganic substrate, like glass, coated on a first surface with a single layer of dielectric material and a second surface with a layer of either a highly reflective metal film or dielectric material. Preferably, the dielectric material exhibits a high reflectivity, which stems from the real part of the refractive index of the material. More preferably, the refractive index of the dielectric material is appreciably higher than that of the transparent substrate. Further, the dielectric material has a low absorption, and its refractive index and absorption are relatively constant over the spectrum of visible wavelengths. Significantly, the polarizer is configured to produce a xe2x80x9cquasi-straight path,xe2x80x9d such that a light ray exiting the substrate is substantially parallel with a corresponding incident light ray entering the substrate. In other words, a light beam exiting the polarizer maintains the same, original direction as the corresponding incident light beam entering the polarizer (See, FIG. 4).
In another aspect, the invention relates to a visible light polarizer including a transparent substrate having two opposite planar coated surfaces and two opposite edges (an entrance and exit edge), cut and polished to be parallel to each other, the plane of the entrance edge being cut and polished to form an angle equivalent to the Brewster angle relative to the plane of the surface of the substrate having a film of dielectric material, the plane of the second edge being cut and polished to form an angle equivalent to the Brewster angle relative to the plane of surface of the substrate having a fully reflective metal film, such that the distance between the planes of the two edges is a function of the type dielectric material used, the thickness of the substrate.
In a further aspect, the invention relates to a bi-directional polarizer characterized in that equal polarizer performance is obtained regardless of the direction on light entry.
In still another aspect, the invention relates to visible light polarizer designed to have a light path that has the incident light enter the first edge and travel through the transparent substrate prior to striking the dielectric film on one surface of the substrate. The inventive light path design pairs the index of refraction of the transparent substrate with the higher index of refraction of the dielectric film. Since the refractive index of the transparent substrate is greater than the refractive index of air, this inventive pairing of the refractive index of the substrate and the refractive index of the dielectric film results in superior polarization relative to pairing of the index of refraction of air with the index of refraction of said first film.
The preferred optical polarizer of the present invention is a miniaturized polarizer made by assembling several polarizer units as described below.